Challenges of considering saliency ratio in design optimization of IPM machines

This paper investigates the challenges of including saliency ratio as an objective function in the design optimization of interior permanent magnet (IPM) machines. The saliency ratio is the ratio between q- and d-axis inductances. Improvement of the reluctance torque component and self-sensing capabilities are directly related to the saliency ratio of the IPM machine. Thus, the maximization of the saliency ratio could be a design objective in the multi-objective design optimization process. The paper shows that assumptions made in conventional methods reduce the accuracy of estimated inductances which deviate the optimization process to a local optimum design. On the other hand, improved accuracy in the method to determine the IPM machine's inductances significantly increases the computational costs in the design optimization process. Motivated by the above challenges, this paper proposes a method to estimate the saliency ratio using magnetic energy in the design optimization of IPM machines. The energy method simultaneously increases the accuracy of inductance calculation and decreases the computational costs. The method considers the saturation of iron cores, cross-magnetization effects and the variation of inductances with respect to rotor positions. A procedure to implement the energy method in a multi-objective optimization algorithm is presented. The saliency ratio optimization using the conventional and the energy method are compared to the experimentally measured values to demonstrate the effectiveness of the proposed approach. A fractional-slot concentrated-wound (FSCW) IPM machine was considered for this study.